Stress and Strain Measurement
Strain, e, is a dimensionless response to stress expressed as a fraction, e=ΔL/Lo, where Lo is the original length of the object and ΔL is the change in length of the object when stress is applied. Stress, s, is a measure of force per unit area given by F/A where F is the force being applied and A is the area it is being applied to. Because stress cannot be measured directly in practice, strain is measured instead. The stress in an object is related to the strain by the Young's Modulus, E, which is given by the following relationship:
  E  =            s      /              e        el              =                            (                      F            /            A                    )                          (                      Δ            ⁢                                                  ⁢                          L              /                              L                o                                              )                    =              stress        /                  strain          .                    
Knowing the Elastic Modulus of a given material, the stress in the material can be determined by measuring the strain. Traditionally, stress and strain measurements have been accomplished by a number of different methods. Some of these methods are described below:
Photoelastic Techniques
Photoelastic techniques are optical techniques for detecting stress and strain that exploit the photoelastic properties of certain materials. The speed of propagation of light in transparent materials is generally slower than in a vacuum or in air. The ratio of the speed of light in a given material to the speed of light in a vacuum is called the index of refraction of that material. In homogeneous materials, the index of refraction is constant regardless of the direction of propagation or plane of vibration of the light. In other materials, strain in the material causes the index of refraction to change depending on the direction of propagation of light. These materials, which can be optically isotropic when unstrained, become optically anisotropic when strain is present.
Materials that become optically anisotropic when stressed are known as photoelastic materials. The change in index of refraction relative to index axis in the material can typically be related to the stress and strain in the material by observing and quantifying the photoelastic effect. The photoelastic effect is caused by alternately constructive and destructive interference between light rays that have undergone relative retardation, or phase shift, in the stressed photoelastic material. When illuminated with polarized light and viewed through a polarizing filter, fringe patterns become visible in the photoelastic material that reveal the overall stress and strain distribution in the part and show the locations and magnitudes of the stresses and strains in the part. A person skilled in the art of photoelastic analysis can interpret and measure these patterns.
Photoelastic analysis techniques can be useful because the results are visual and relatively easy to interpret. However, photoelastic analysis has traditionally been limited to R&D and laboratory testing applications because of a number of factors. First, the coatings are difficult to put on and do not lend themselves well to application to production parts. Secondly, specialized equipment is required to conduct the tests and to make the fringe patterns reveal themselves. The specialized equipment includes a source of polarized light and another polarizing filter to view the test specimen through. The second polarizing filter can be integrated into an instrument known as a polarizer. This equipment can be costly and difficult to find. A second problem with traditional photoelastic analysis techniques is the need to illuminate the part with polarized light. This usually requires a dark room and precludes inspections in the presence of non-polarized ambient light and makes outdoor inspections difficult.
Photoelastic Coatings
Photoelastic coatings are available from companies such as Measurements Group (http://www.vishay.com/company/brands/measurements-group/) in sheet form, and also in a liquid plastic form that is cast onto a part and then bonded on using adhesive. Lam and Ellens disclose a method for applying a photoelastic coating using powder coating techniques (U.S. Pat. No. 6,650,405) that is low cost, easy to apply, and can be used on parts with complex three-dimensional shapes. The application of a photoelastic coating using powder coating methods is much less expensive than applying of traditional photoelastic coatings, and for example is suitable for application to parts that are made in higher quantities for field use. Once powder coated with the photoelastic layer, the parts and structures can then be inspected in service using photoelastic techniques to determine if any strain is present in the part or if any plastic deformation has occurred. Applying a photoelastic layer using powder coating methods can be a useful and low cost method of monitoring parts and can increase public safety by helping with early detection of failures before they become catastrophic.
Structural Monitoring
Structures such as buildings, bridges, airplanes, and other critical structures are prone to failure. Failure can happen with significant loss of life and property, as evidenced by the recent collapse of the I-35 bridge in Minnesota. As such, structural monitoring is becoming an even more important field that can have a significant positive impact on public health and safety.
Prior art approaches to structural monitoring typically involve expensive monitoring devices and expensive monitoring systems. A prior art system involving the use of a linear transducer is disclosed by Arms (U.S. Pat. No. 6,588,282). The transducer has two components that move relative to one another, and a clamping mechanism is used to prevent the shortening of the relative strain between the two components. This system has disadvantages because of the moving parts involved, and the precise orientation required for those moving parts to work. Also, the system requires some sort of electronic system to detect the signal. These factors combine to make this system expensive and less suitable for large scale deployment for structural monitoring.
Another prior-art approach to structural monitoring is disclosed by Brennen (U.S. Pat. No. 6,928,881). Brennen's approach involves the use of strain gauges mounted in a housing along with instrumentation that records and stores the stress levels experienced by the structure. While this approach can give an accurate time history of stresses experienced by a structure, it is prohibitively expensive because of the use of on-board monitoring and storage. The collection of all the historical stress data is overkill and unnecessary. The regular storage and retrieval of all the data is too expensive and requires excessively frequent visits to perform downloads. Also, because data will only be recorded at a certain sampling rate, it is possible with Brennen's system to miss key events such as stress strain peaks. This could lead to false conclusions regarding the safety of the structure because key peak events have been missed. Increasing the sampling rate to try to capture these peak key events means more data needs to be stored. Reducing the sampling rate in order to reduce the amount of data stored only makes the problem of missing key events worse.
Bilder et al, (U.S. Pat. No. 5,534,289) discloses a method of structural crack detection that involves the use of microcapsules in a coating that will burst and change the colour of a second coating layer. The coatings also provide environmental protection for the structural part. This method however can only detect cracks, and the requirement of microcapsules containing coloured dyes makes the method difficult and expensive.
Prior art photoelastic layers are limited in their applicability to structural monitoring because of several reasons. One reason is that photoelastic analysis typically requires a polarized light source, and large structures in the field such as bridges are hard to shield from ambient light and hard to illuminate with polarized light. Another reason is that photoelastic analysis typically requires specialized equipment such as a polarized light source and a detector with a built in polarizer to see the fringe patterns. Photoelastic analysis has also typically been limited to laboratory environments. Traditional photoelastic analysis techniques are not well suited to larger structures and analysis in outdoors situations.
Problems with Prior Art
From the description above, it can be seen that prior art methods of photoelastic analysis and structural monitoring can suffer from one or more of the following problems:                An external source of polarized light is required to inspect structures or devices with photoelastic coatings or properties.        Inspections in daylight cannot be performed without the need to block out the ambient light. Blocking out the ambient light to perform an inspection is not very practical on a structure such as a bridge when the area in question is not very accessible and the bridge is already being illuminated by ambient light from the sun.        Inspections with traditional photoelastic techniques require a polarized light source to illuminate the object being inspected in order to perform the inspection. Ambient light cannot be used. This can make it difficult to perform inspections on objects and structures at a distance, particularly when ambient light is present.        Inspections cannot be made on structures from a distance. For example, inspections cannot be made from ground level on high bridges or overpasses using magnifying optics such as a telescope or telephoto lens on a camera because of the need to illuminate the photoelastic layer or photoelastic-coated monitoring device with polarized light.        Specialized imaging and detection equipment designed for photoelastic analysis is required to perform the inspections. Regular imaging equipment such as digital cameras cannot be used to perform the inspections. This increases the cost of inspections.        Two polarizing filters are typically required with traditional photoelastic techniques. These traditional photoelastic techniques typically require a light source, a polarizing filter to polarize the light from the light source, and a second polarizing filter to view the part through.        Current methods for monitoring structural components such as bridges, buildings, and components such as aircraft landing gear are expensive and do not allow an inspector to determine whether the structure has experienced excessive stress strain levels or not.        Current stress and strain detection and analysis techniques including photoelastic techniques, strain gauges, and other techniques are not well adapted to use in the field, particularly for long-term low-cost monitoring. Monitoring of stresses and strains in the field using existing instruments and methods typically requires constant monitoring and logging of data to capture the stress strain history of the structure and part. This can be problematic and expensive.        Without some sort of data-logging, many existing stress and strain detection techniques are not able to communicate to the inspector that a certain level of stress and strain has been experienced by the structure or part even when the structure is no longer experiencing that stress level.        Existing methods for detecting when stress and strain levels have been exceeded can be expensive to build, to install, and to perform regular inspections on.        
In summary, current stress strain detection techniques suffer from a number of problems that make their use in structural monitoring problematic and have prevented the growth of structural monitoring solutions for improving public safety. In particular, photoelastic analysis has been prevented from gaining more widespread acceptance in structural monitoring because traditional photoelastic techniques have not been appropriate for use in structural monitoring.